Monoclonal antibodies to heparan sulfate inhibit the formation of thrombin-antithrombin III complexes

As components of the cell surface and extracellular matrix, heparan sulfate proteoglycans play an important role in the function of most organs, including the vasculature. Autoimmunity to heparan sulfate (HS) glycosaminoglycan epitopes may play a role in tissue injury. Tight skin (TSK) mice suffer a disease resembling scleroderma, an autoimmune disease associated with alterations in the extracellular matrix of the skin and other organs, as well as vascular injury. Studies of autoimmunity to HS were performed in TSK mice employing a solid-phase radioimmunoassay for the detection of anti-HS antibody. An increase in the clonal frequency of hybridomas secreting anti-HS antibody was noted in TSK mice compared to that in control mice. Liquid-phase competitive inhibition specificity studies of IgM and IgG anti-HS monoclonal antibodies demonstrated little cross-reactivity with other glycosaminoglycans. Some cross-reactivity, albeit at lower affinity, with phosphorylated antigens including DNA, cardiolipin, and RNA polymerase were noted. Negatively charged sulfate groups and carboxyl groups played a role in the immunodominant site. Monoclonal antibodies to HS inhibited the binding of HS to antithrombin III and the formation of thrombin-antithrombin III complexes. These results demonstrate autoimmunity to HS in TSK mice. The results also indicate that HS is a high-affinity antigen of pathological significance for anti-DNA and anti-phospholipid antibodies. Thrombosis induced by polyclonal anti-phospholipid antibodies may be mediated by a subset of these antibodies with high affinity for HS. Anti-HS antibodies may promote a procoagulant state by the blockade of HS binding to antithrombin III, inhibiting the accelerated formation of thrombin-antithrombin III complexes.